1. Field of the Invention
This invention relates to solids-liquid separating centrifuges of the continuous type in which a bowl, imperforate or perforate, and a conveyor are rotated about a common axis in the same direction but at a differential speed. More particularly, the invention concerns the provision of such centrifuges with a system for sensing the existence of excessive torsional vibration and providing a control initiating signal.
2. Description of the Prior Art
Centrifuges of the type concerned, particularly when used with certain types of materials, are subject to relative rotational or torsional vibration between the bowl and conveyor, a condition commonly referred to as "chatter". The high frequency torque fluctuations produced by chatter, typically 20 to 60 cycles per second, are of large amplitude at high throughputs with resultant strain on and fatigue of the differential gearing commonly used between bowl and conveyor, and torque overload shearpins. The consequent shortened life of these components is expensive both because the gears are costly to replace, and because of machine down time while either gears or shearpins are replaced. U.S. Pat. No. 3,685,722 discloses a flexible connection system between the conveyor and the gearing which desirably reduces the amplitude of chatter but even with machines having this improvement chatter may still be a problem at high throughputs, which a user may wish to obtain.
Despite extensive studies, the exact causes of chatter may not be fully known. Since excessive chatter is most frequently encountered with slurries of solids of a sticky nature such as starch, sewage sludge and PVC, chatter is generally assumed to be caused by irregular conveying of the solids in a series of stick-slip motions. The phenomenon is not necessarily predictable even for a given machine operating at given parameters on a given slurry, since it may suddenly occur due, apparently, to a fluctuation in slurry concentration or content or feed rate. Once excessive chatter is encountered, it is usually necessary to reduce throughput drastically in order to eliminate chatter or reduce it to acceptable proportions.
It has been known in the prior art to translate chatter into an electrical output of correspondingly fluctuating amplitude by means of appropriately located strain gages, piezoelectric accelerometers or the like, this fluctuating output being observable on an oscilloscope. Such a system, used mainly in laboratory studies, has been used to monitor chatter in centrifuges operating in production in the field. Such a system is expensive, since it requires substantially constant attention of an operator to the oscilloscope, so that he can take appropriate steps to reduce throughput, usually by reducing feed rate, when the amplitude of the observed fluctuations of the wave indicative of chatter becomes excessive in his judgment. In most cases, centrifuge operators either take the consequences of excessive chatter at desired high throughputs, or maintain a much lower throughput than would otherwise be desirable at which excessive chatter is unlikely to occur.